Heretofore, there have been proposed a number of amorphous semiconductor films such as an amorphous deposited film composed of an amorphous silicon material compensated with hydrogen atoms (H) or/and halogen atoms (X) such as fluorine or chlorine atoms [hereinafter referred to as "A-Si(H,X)"]. Some of such films have been put to practical use as an element member in semiconductor devices, electrophotographic photosensitive devices, image input line sensors, image pickup devices, photovoltaic devices, other various electronic and optical devices. Several of them have now been put into practice.
It is known that such semiconductor deposited films may be obtained by means of a DC glow discharge decomposition process, a high frequency glow discharge decomposition process or a microwave glow discharge decomposition process, that is, a process of forming a deposited film on a substrate of glass, quartz, heat-resistant resin, stainless steel or aluminum by decomposing a film-forming raw material gas with a glow discharge using a DC energy, a high frequency energy, or a microwave energy. In recent years, the public attention has been focused on the glow discharge decomposition method using a microwave energy (that is, MW-PCVD method) at industrial level since the MW-PCVD method is remarkably advantageous over other methods with the points that film deposition rate is high and utilization efficiency of a film-forming raw material gas is high.
Along with this, there have been proposed various MW-PCVD apparatuses for practicing said MW-PCVD method.
In Japanese Laid-open Patent Application No. 60-186849, there is disclosed a MW-PCVD apparatus utilizing the above-mentioned advantages. This apparatus includes a discharge space wherein a plurality of cylindrical substrates are arranged to surround a microwave energy-introducing means, thereby highly enhancing the gas utilization efficiency.
In addition, in Japanese Laid-open Patent Application No. 61-283116 (U.S. Pat. No. 4,619,729, issued Oct. 28, 1986), there is disclosed an improved microwave system for fabricating a semiconductive member. This method lies in improving characteristics of a deposited film wherein there are provided electrodes in a plasma space for controlling the potential of plasma as formed, and a desired voltage is applied between the electrodes to deposit a film while controlling ion impact of the deposition species.
Now, a typical example of the foregoing MW-PCVD apparatus has such constitution as shown FIGS. 6(A) and 6(B). FIG. 6(A) is a schematic explanatory view illustrating the constitution of the known MW-PCVD apparatus, and FIG. 6(B) is a schematic cross section view of the apparatus shown in FIG. 6(A) by X--X line.
In FIGS. 6(A) and 6(B), reference numeral 601 indicates a film-forming chamber having a vacuum tight structure. Reference numeral 602 indicates a microwave introducing window made of a dielectric material capable of efficiently transmitting microwave into the film-forming chamber. Said material of which the microwave introducing window is made is quartz glass, alumina ceramics or the like. Reference numeral 603 designates a waveguide connected to a microwave power source (not shown) through a stab tuner (not shown) and an isolator (not shown). Reference numeral 604 indicates an exhaust pipe which is provided with the film-forming chamber 601, and it is connected through an exhaust valve (not shown) to an exhaust device (not shown). Reference numeral 605 indicates cylindrical substrates on each of which a deposited film is to be formed, and reference numeral 606 indicates a discharge space surrounded by the substrates 605.
The formation of a deposited film in this apparatus is carried out in the following manner.
The film-forming chamber (601) is evacuated through the exhaust pipe (604) by means of the exhaust device (vacuum pump) (not shown) to such an extent that the pressure in the film-forming chamber is controlled to be 1.times.10.sup.-7 Torr or below. Subsequently, each of the substrates (605) is heated to and maintained at a temperature necessary for film deposition by the use of external heaters (612). Film-forming raw material gases are introduced into the film-forming chamber (601) through a gas feed means (not shown), for example, in the case where an amorphous silicon deposited film is formed, film-forming raw material gases such as silane gas and hydrogen gas are introduced into the film-forming chamber. Simultaneously, the microwave power source is switched on to generate a microwave having a frequency of not less than 500 MHZ, preferably 2.45 MHz, followed by transmission through the waveguide (603) and the microwave introducing window (602) into the film-forming chamber (601). In this way, the film-forming raw material gases are excited by means of the microwave energy in the discharge space (606) surrounded by the cylindrical substrates (605) and they are decomposed, thereby causing the formation of a deposited film on each of the substrates. During this process, the substrates (605) are rotated. As a result, there is formed a deposited film over the entire surface of each of the substrates.
According to the above film-forming process, a certain deposition rate enables one to obtain a deposited film having practically appreciable characteristics and uniformity. However, if the deposition rate becomes higher, it is difficult to invariably and stably obtain a deposited film with a uniform film quality having satisfactory optical and electric characteristics in high yield particularly in the fabrication of a deposited film of a large area as required, for example, for an electrophotographic photosensitive member. This is a serious problem to solve.
More particularly, in order to form a deposited film on a substrate of large area at a high deposition rate while keeping a high utilization efficiency of a film-forming raw material gas, it is necessary to introduce a great quantity of microwave energy into the film-forming chamber. For this purpose, there is used a waveguide having a dielectric window excelling in microwave transmission. In this case, the microwave energy density in the vicinity of the microwave introducing means in the film-forming chamber becomes far greater than those in other portions. This entails irregularities in the thickness and the quality between a deposited film formed on the part of the surface of a substrate located near the microwave introducing means and a deposited film on the remaining part thereof, with a lowering of the characteristics of the deposited film. Moreover, a film which is formed as relatively thick in the vicinity of the microwave introducing means will often peel off and the thus removed films will often deposit on a film formed on another part of the substrate, thus causing defects for the film obtained.
In order to eliminate the above problems, there has been made a proposal by Japanese Patent Publication No. 61-53432 wherein an auxiliary substrate is provided with the substrate in the vicinity of the microwave introducing means. The method according to this proposal is effective in other PCVD processes than MW-PCVD process. That is, in the case of the MW-PCVD process using microwave plasma wherein microwave is introduced through a waveguide, film separation from the auxiliary substrate cannot be prevented completely, and therefore, the method according to said proposal is not sufficient to eliminate the foregoing problems. In addition, in the case of the film-forming process to be practiced in the apparatus shown in FIGS. 6(A) and 6(B), the microwave energy density is relatively high in the vicinity of the microwave introducing means (602, 603), and because of this, there will be deposited a film also on the surface of the microwave introducing window in the film-forming chamber with a thickness being several times thicker than that of a film deposited on the surface of each of the cylindrical substrates. And the film thus deposited on the surface of the microwave introducing window results in peeling off in pieces which contaminate in a film to be deposited on the surface of each of the cylindrical substrates. In this respect, it is difficult for the resulting deposited film according to this method to be completely free from the problems relating to defects.
Further, the apparatus shown in FIGS. 6(A) and 6(B) is accompanied with other problems with respect to the thickness and the uniformity in characteristics for a deposited film to be formed. That is, in this apparatus, film-forming raw material gases are decomposed with the action of microwave energy to cause the formation of a deposited film. The unreacted gases and the gases formed as a result of the reaction are exhausted through spaces of a rectangular form established between the cylindrical substrates (605) to outside the discharge space (606). At that time, part of those gases is exhausted from the spaces between the end of each cylindrical substrate (605) including the auxiliary substrate (611) and the waveguide (603). The gas flow which caused in this case makes a flow of gas along the direction of the generating line on the surface of each cylindrical substrate (605), thus causing unevenness in the thickness and characteristics for a film to be deposited.
One of the reasons why a deposited film having good characteristics cannot be obtained particularly at high deposition rate in the conventional film-forming process is that a difficulty is involved in the temperature control of a substrate during the plasma discharge. That is, in order to heighten the deposition rate, it is necessary to raise the flow rate of a film-forming raw material gas and the discharging microwave power. For instance, in order for the deposition rate to be 50 .ANG./sec. or more, a discharging microwave power of 1 KW or more is necessitated. And when the discharge is continued with such high discharging microwave power, the substrate temperature accordingly continues to increase because of heat generated by the microwave applied. In view of this, it necessitates to properly control the substrate temperature during the film formation. For properly controlling the substrate temperature, there are known several manners: a manner wherein a heater installed in a substrate holder is turned off simultaneously with commencement of the plasma discharge, a manner wherein a heater installed in a substrate holder is provided with a cooling pipe and is used not only as a heating means but also as a cooling means, and a manner wherein a vacuum container serving only to heat a substrate is provided and after the substrate being heated therein, it is vacuum-transferred to a film-forming chamber. However, satisfactory effects cannot be obtained by any of these manners.
By the way, it is well known that the characteristics of a deposited film prepared according to the conventional MW-PCVD process are greatly influenced by the substrate temperature during the deposition, e.g. when a deposited film comprised of A-Si(H,X) for photosensitive drum is formed during which the substrate temperature is over a certain level, hydrogen atoms in the deposited film are dissociated and this makes the resulting photosensitive drum to be poor in the charging properties in an electrophotographic image-forming process.
The present inventors have found that there are still problems even in the case of the MW-PCVD process wherein ion impacts of deposition species are controlled in order to improve the characteristics of a deposited film using such MW-PCVD apparatus as shown in FIGS. 5(A) and 5(B) which has been established by the present inventors, wherein a bias electrode is provided in the discharge space in combined use with microwave plasma discharge, and the electrode is applied with a desired DC voltage. The MW-PCVD apparatus shown in FIGS. 5(A) and 5(B) is of the same constitution as the conventional MW-PCVD apparatus shown in FIG. 6(A) and FIG. 6(B) except that there is provided a bias electrode 511. That is, in addition to the microwave plasma, the energy of ion impacts on the substrate after acceleration of the ions with an electric field contributes to heat the substrate. As a result, the substrate eventually becomes very high in temperature as the discharge time is prolonged, and because of this, the resulting deposited film is apt to become ununiform in the characteristics. This process includes another factor to make the resulting deposited film to be lacking in the uniformity of the characteristics because of easy occurrence of ununiformity in ion impacts. That is, the microwave introducing means is provided near the end portion of the cylindrical substrate in the apparatus in which this process is to be practiced as described above and because of this, the ion density in the vicinity of the microwave introducing means is inevitably increased. This causes ununiformity in the ion impacts on the surfaces between the region in the vicinity of the microwave introducing means and the other region, causing the characteristics of the resulting deposited film to be ununiform. The ununiformity of the ion impacts becomes more pronounced with increasing microwave discharging power. As above described, the current technical situation in the formation of a functional deposited film by means of MW-PCVD process is that it is difficult to stably form a desirable functional deposited film of large area having a uniform thickness and excelling in evenness for the characteristics as desired.